1. Field of the Invention
The present invention relates to a laser light source device, particularly to a technology of controlling the temperature of a laser light source device that uses a wavelength conversion element.
2. Description of the Related Art
Recently, a technology of using a laser light source as a light source device for a monitor and a projector has been suggested. In contrast to a UHP lamp that has been conventionally used as a light source, the spectrum (spectral characteristic) of the laser light source does not spread out and exhibits high color purity. The laser light source is therefore advantageous because it has a wide color gamut and a long life and glows immediately after it is turned on.
Examples of laser light sources include laser diodes (semiconductor lasers) and second-harmonic generation (SHG) lasers. Although a laser diode is less expensive than an SHG laser, laser diodes of some colors have not yet been turned to practical use for a monitor or the like that requires various colors (with three basic RGB colors). Furthermore, laser diodes of colors like green that recently become available are costly.
In reality, to supply laser light of a desired wavelength by using a readily available general-purpose laser light source, the wavelength of a fundamental laser such as an SHG laser should be converted for a laser light source of a color that is not in practical use or a color that is in practical use but is still expensive. It has been known, however, that the wavelength conversion efficiency of the SHG element tends to decrease due to the refractive-index distribution that varies in accordance with the change of temperature. To improve the efficiency of the laser light of a laser using the SHG element and ensure a constant amount of light, a technology of controlling the temperature of the SHG element has been developed (see, for example, Japanese Patent Application Laid-open No. 08-101416 and Japanese Patent Application Laid-open No. 2008-164900).
Among the conventional technologies, Japanese Patent Application Laid-open No. 08-101416, for example, suggests a technology with which control is performed to keep the SHG output power constant and thereby realize a reduced size and low power consumption by converting the output power in accordance with a temperature dependency table that shows the dependency of the SHG output power with respect to the change of the measured temperature of the laser element, and storing in advance the SHG output power with respect to the change of temperature.
Furthermore, Japanese Patent Application Laid-open No. 2008-164900 suggests a device that controls the temperature in real time based on the result of measuring the temperature of an SHG element and a controlling method thereof. When the laser light source is actually controlled by use of the conventional technologies, however, the temperature may vary more than expected, or it may not be sufficiently controlled if variations of the laser elements are too large.
According to the method of Japanese Patent Application Laid-open No. 08-101416, the output power is not always suitably controlled only with the temperature dependency table that is meant to correct the thermal property of the laser element because the SHG element also has its own thermal property. In addition, according to the method of Japanese Patent Application Laid-open No. 2008-164900, because the temperature is measured in the SHG element and its surrounding area, the optical output is not controlled to be constant with respect to the change of temperature of the LD chip that may occur in accordance with changes of the drive current or drive signal to the laser element, and thus it is difficult to perform control to make the optical output constant. For this reason, the efficiency cannot be maintained high because the optimal temperature of the laser element varies in accordance with the conditions of driving the laser element.